In the fabrication of semiconductor wafers several process steps require contacting the wafers with fluids. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. Often the chemicals utilized in these steps are quite hazardous in that they may comprise strong acids, alkalis, or volatile solvents.
The equipment conventionally used for contacting semiconductor wafers with process fluid consists of a series of tanks or sinks into which cassette loads of semiconductor wafers are dipped. Such conventional wet processing equipment poses several difficulties.
First, moving the wafers from tank to tank may result in contamination, which contamination is extremely detrimental to the microscopic circuits which the fabrication process creates. Second, the hazardous chemicals and deionized water which are used have to be regularly replaced with new solutions, usually introduced to the tank by bottle pour, chemical distribution or from building facilities in the case of deionized water. The chemicals generally are manufactured by chemical companies and shipped to the semiconductor manufacturing plant. The chemical purity is thus limited by the quality of the water used by the chemical manufacturers, by the container used for shipping and storing the chemical and by the handling of the chemical.
Moreover, as chemicals age, they can become contaminated with impurities from the air and from the wafers. The treatment of the last batch of wafers prior to fluid rejuvenation may not be as effective as treatment of the first batch of wafers in a new solution. Non-uniform treatment is a major concern in semiconductor manufacturing.
Some of the fluid contact steps of semiconductor manufacture include removal of organic materials and impurities from the wafer surface. For example, in the manufacture of integrated circuits, it is customary to bake a photoresist coating onto a silicon wafer as part of the manufacturing process. This coating of photoresist or organic material must be removed after processing.
Generally, a wet photoresist strip process is performed by a solution of sulfuric acid spiked with an oxidizer of either hydrogen peroxide or ozone. This process is referred to in U.S. Pat. Nos. 4,899,767 and 4,917,123, issued to CFM Technologies. However, there are many disadvantages to using a solution of sulfuric acid and an oxidizer to strip photoresist from wafers during semiconductor manufacture. First, the by-product of the resist strip reaction when hydrogen peroxide is used as the oxidizer is water, which dilutes the concentration of the bath and thereby reduces its ability to strip photoresist. Second, this process operates at a high temperature, generally between 80.degree. and 150.degree. C., typically above about 130.degree. C., which mandates the use of special heat resistant materials and components in order to house, circulate and filter the solution, as well as requires extra energy to conduct the cleaning process. Third, the solution is hazardous to handle and dispose of and expensive to manufacture, transport and store.
Moreover, due to the build-up of impurities both dissolved and undissolved in the process bath, the solution must be changed periodically. Typically, the interval for chemical change out is about every eight hours. Because the chemical adversely affects the drain plumbing, the solution must be cooled to less than about 90.degree. C. prior to disposal. Thus, use of this photoresist stripping process requires either the use of additional tanks to contain the hot solution or the shut down of the process station during the chemical change period, reducing wafer throughput and increasing cost of ownership.
Finally, after use of a sulfuric acid solution for removal of photoresist, the wafers must be rinsed in hot deionized water since sulfate residues may crystallize on the wafer during processing causing process defects.
Another process often utilized for the removal of organic and metallic surface contaminants is the "RCA clean" process which uses a first solution of ammonium hydroxide, hydrogen peroxide, and water and a second solution of hydrochloric acid, hydrogen peroxide, and water. The RCA cleaning solutions typically are mixed in separate tanks. The wafers are first subjected to cleaning by the ammonium hydroxide solution, then are moved to a rinse tank, then to a tank containing the hydrochloric acid cleaning solution, and then to a final rinse tank. This process, like the sulfuric acid process, has the disadvantage of using strong chemicals. Moreover, the wafers are exposed to air during the transfers from tank to tank, allowing for contamination. Finally, the use of peroxide may cause the wafers to suffer aluminum contamination from the deposition of aluminum in the high pH ammonium hydroxide solution which is not totally removed in the hydrochloric solution.
Various approaches have been taken to improving the processes and equipment used to treat semiconductor wafers with fluid. These attempts to improve on present processes generally involve either a change in equipment or a change in the process chemicals.
One approach to eliminating problems with contamination of wafers during fluid treatment is disclosed in U.S. Pat. Nos. 4,778,532, 4,795,497, 4,899,767 and 4,917,123. These patents describe an enclosed full-flow method and apparatus which allows the process fluids to flow sequentially and continuously past the wafers without movement or operator handling of the wafers between processing steps. However, these patents still teach the use of hazardous chemicals to perform the fluid treatment and cleaning of the wafers. Moreover, the equipment needed for the enclosed apparatus is limited in wafer throughput since all process sequences are performed in the same vessel with concentrated solutions.
U.S. Pat. No. 4,899,767 teaches the use of a separate mixing tank for preparing the sulfuric acid and oxidizer solution, which solution must then be delivered into the treatment tank. The reason for the separate tank is to eliminate the possibility of an explosion due to pressure build up from the decomposition of hydrogen peroxide into oxygen and water.
U.S. Pat. No. 5,082,518, issued to SubMicron Systems, Inc., describes a different approach to improving the sulfuric acid and oxidizer process of cleaning semiconductor wafers. The system in this patent provides a gas distribution system which includes a sparger plate with diffusion holes for distributing gas throughout the bath in the tank. Thus, rather than using a separate tank for mixing as in U.S. Pat. No. 4,899,767, the SubMicron patent provides an apparatus which distributes ozone directly into the treatment tank containing sulfuric acid. It has been found, however, that this diffusion system suffers several disadvantages. First, the efficiency of ozone distribution and absorption into the water is lessened by the large bubbles of ozone produced by the apparatus. The amount of ozone absorbed is important to its ability to react with the sulfuric acid to remove organic materials from the wafers. Moreover, the type of diffusing element described in U.S. Pat. No. 5,082,518 is believed to not uniformly distribute ozone throughout the tank. Finally, as with previous attempts to improve cleaning processes for wafers, hazardous chemicals are required, creating handling and disposal problems.
An approach to eliminating the problem of the use of hazardous chemicals is set forth in Ohmi et al, J. Electrochem. Soc., Vol. 140, No. 3, March 1993, pp. 804-810, which describes the use of ozone-injected ultrapure water to clean organic impurities from silicon wafers at room temperature. However, this process also suffers several disadvantages. Ohmi et al provides only a process for the removal of a very thin layer of organic material, i.e., a layer of surfactant left from the lithography process. The process described by Ohmi et al could not remove photoresist in a reasonable time frame. The process was intended for, and works on organic contamination layers of less than 50 Angstroms. It is too slow to work on organic contamination layers of 50-250 mils. Thus, a process which can quickly and effectively remove organic materials of all thicknesses from semiconductor wafers without the use of hazardous chemicals is still not available in the art.
A process for the removal of organic materials during semiconductor manufacture which can avoid the foregoing problems while providing effective removal of organic materials would be of great value to the semiconductor industry. Further, an apparatus for conducting such a process which eliminates the need for multiple tanks would also be of great value to the industry.
Accordingly, it is an object of the present invention to provide a process for the removal of photoresist or other organic materials from semiconductor wafers which does not use hazardous chemicals.
It is still another object of the present invention to provide a process which does not require cool down tanks or idle stations due to chemical change out.
It is still another object of the present invention to provide a system for the fluid treatment of semiconductor wafers in which more than one treatment can be conducted without moving the wafers to a separate tank.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.